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Every time a phone connects to a tower, it uses something that did not exist in any laboratory until the 1880s: an electromagnetic wave made on purpose and detected somewhere else. James Clerk Maxwell's equations predicted such waves. Heinrich Hertz made them and measured them. This post explains how, sorts out a date you may see written differently, and works through the numbers.
Who was Heinrich Hertz?
A press release from the Karlsruhe Institute of Technology (KIT), drawing on its archive, says Hertz did research and taught at the Technical High School in Karlsruhe from 1885 to 1889, his first post as a full professor, at 28. He then moved to Bonn, where he died of sepsis in 1894 at the age of 36. He was born, and is buried, in Hamburg.
When did Hertz first detect electromagnetic waves?
NCERT's Class 12 chapter on electromagnetic waves says "Hertz, in 1885, experimentally demonstrated the existence of electromagnetic waves." The KIT archive is more specific: on 11 November 1886, in the lecture hall now named after him, Hertz observed the propagation of an electromagnetic wave for the first time. Use NCERT's wording in a board answer, but know that 1885 is the year he arrived at Karlsruhe and the experiments ran from 1886 to 1888.
How did his apparatus work?
KIT describes the set-up. The emitter was an oscillating dipole with a spark gap, driven by an induction coil. The receiver was a wire ring with a small gap. When a spark jumped at the emitter, a tiny spark appeared across the gap in the ring some distance away, with no wire between them.
Hertz told the story himself in the introduction to Electric Waves, the 1893 English translation by D. E. Jones of his papers. A happy chance in the spring of 1886, while using a pair of induction spirals in the Karlsruhe instrument collection for lectures, led him into the research. His first major paper, On very rapid electric oscillations, appeared in Annalen der Physik in 1887.
How did he show the waves behave like light?
In the paper translated as On Electric Radiation (Annalen der Physik, 1889), Hertz used shorter waves and placed the emitter and receiver at the focal lines of two concave mirrors about 2 metres high. That gave him a beam, and he tested it one property at a time:
Straight-line travel. A zinc screen 2 metres high in the path made the receiver's sparks disappear completely. So did an assistant walking through the beam. A wooden door did not stop it.
Polarisation. He stretched copper wires 1 mm thick and 3 cm apart across a frame 2 metres wide. With the wires at right angles to the focal lines, the screen hardly affected the sparks. With the wires parallel, it stopped the ray completely, "just as a tourmaline plate behaves towards a plane-polarised ray of light."
Refraction. He had a prism of hard pitch cast, weighing about 12 hundredweight, with a refracting angle of about 30°. The sparks peaked at a deviation of about 22°.
Worked example: frequency from Hertz's wavelength
In On Electric Radiation, Hertz used standing waves in front of a wall and set 33 cm as a closer value of the half-wavelength. KIT's summary puts his waves at 30 to 100 cm, with frequencies from about 300 to 1000 MHz.
Question. Find the frequency for a half-wavelength of 33 cm, taking c = 3.0 × 10⁸ m/s.
Nodes in a standing wave are half a wavelength apart, so λ = 2 × 0.33 = 0.66 m.
f = c / λ = 3.0 × 10⁸ ÷ 0.66 ≈ 4.5 × 10⁸ Hz, about 450 MHz.
That lies in the radio and microwave range of the spectrum in your NCERT chapter. The classic mistake is to treat the node spacing as a full wavelength, which doubles the frequency. Our post on the electromagnetic spectrum lists the bands.
Worked example: Hertz's pitch prism
Hertz reported a refracting angle of 30° and a deviation of 22° near minimum deviation, and said this corresponds to a refractive index of 1.69. Check it with the prism formula n = sin((A + δ)/2) / sin(A/2).
(A + δ)/2 = (30° + 22°)/2 = 26°, and sin 26° ≈ 0.4384.
A/2 = 15°, and sin 15° ≈ 0.2588.
n ≈ 0.4384 ÷ 0.2588 ≈ 1.69.
His arithmetic holds. He also warned readers not to read much into the exact value, since his method was not accurate and the pitch was impure. Both the JEE (Main) 2026 syllabus and NEET-UG list electromagnetic waves and the electromagnetic spectrum.
What can a student take from Hertz's way of working?
Note side effects, then stay on the main question. In 1887 Hertz noticed that ultraviolet light made sparks jump more easily, and reported it in a separate paper. In Electric Waves he says that once he knew it was an ultraviolet effect, he put it aside to return to his main question. That side effect was the photoelectric effect.
State the theory in its testable form. In the same introduction he wrote: "Maxwell's theory is Maxwell's system of equations." What he tested was what the equations predicted, not anyone's picture of them.
Report your limits. He gave his refractive index and in the same breath said how far to trust it.
How an oscillating charge sends out a wave is much easier to see than to read. The Fermi Simulation Lab has interactive simulations for ideas easier to see than read. For the theory Hertz confirmed, read about Maxwell's equations and light.
Try it now: find the frequency of a Hertzian wave 1 metre long.
